Ok, here's a real world possible scenario demonstrating what I mean:
Let's say the TIU's 2 Active channels are each being powered by an output from 2 different phased post war transformers, each capable of delivering >10 Amps. Following best practices, the transformers' Commons are connected together and each transformer Hot output passes through an instant type breaker on its way to the TIU input. So far so good, if either circuit's current exceeds 10 Amps its breaker trips.
Everything works fine until one day the (grandson/daughter, dog, cat or rodent) accidentally disconnects, chews through or otherwise interrupts one of the Common connections returning to only one of the TIU's outputs. The next time the layout is powered up and significantly more than 10 Amps is drawn by the trains, puff magic smoke from the TIU.
~ 20 Amps of current would then be flowing through the one remaining TIU channel Common that's still connected. If the TIU output Commons were also individually protected by 10 amps max instant breakers, this scenario could be prevented.
If the TIU's Common circuitry path can handle more than 10 amps (unclear), then all of this is a mute point.
